A conventional image forming apparatus typically includes a developer refreshing mechanism that supplies new developer consisting of toner and carrier particles, and discharges old developer, to maintain the quality of the developer.
In an image forming apparatus that uses a plurality of developing units for an image forming process, the above-described developer refreshing process needs to be carried out for each of the developing units. The developer refreshing mechanism carries out the developer refreshing process for a developing unit when a predetermined condition is met, e.g., when the number of sheets printed using toner in the developer supplied by the developing unit reaches a predetermined threshold value.
Since the utilization rate of the developer in each of the developing units varies, the developer refreshing mechanism carries out the developer refreshing process for each of the different developing units independently. However, if the utilization rate of the developer in the developing units is similar, the timings for carrying out the refreshing process for the developing units may be close to each other. In such a situation, the refreshing process for one developing unit may start during or just after the refreshing process for another developing unit. Since the image forming function of the image forming apparatus cannot be used during the refreshing process, the user of the image forming apparatus may need to wait a long time until the refreshing processes for all of the developing units have completed.